This guide covers the planning system in England only. Scotland, Wales and Northern Ireland have different rules. Even within England, a listed building, conservation area, Article 4 direction or earlier alterations can change what is allowed.
When might an extension be permitted development?
Subject to the full conditions and exceptions, the Planning Portal explains these common limits:
| Extension | Common England permitted-development position |
|---|---|
| Single-storey rear | Up to 3m beyond the original rear wall for many non-detached houses, or 4m for detached houses, subject to all other limits. |
| Larger single-storey rear | Eligible houses may extend up to 6m (non-detached) or 8m (detached) via a separate prior-approval process. |
| Side extension | Must generally be single-storey, no more than 4m high and no wider than half the original house. |
| Two-storey side extension | Requires householder planning permission under the normal England rules. |
These are simplified examples, not a permission decision. Overall height, eaves height, boundaries, designated land, original-house dimensions and prior enlargements must all be checked. A combined side-and-rear extension can be subject to both sets of restrictions.
What is prior approval?
Prior approval is a separate procedure relevant to certain larger single-storey rear extensions. It involves notifying the local planning authority and allowing neighbours to raise concerns. You cannot simply assume that an eligible depth means work can start immediately. The Planning Portal's prior-approval guide explains the process.
What if your home is not a straightforward house?
- Flats and maisonettes do not have the same householder permitted development rights.
- Listed buildings and conservation-area properties may face additional restrictions or require other consents.
- Previous extensions are relevant when calculating the total enlargement.
- Local restrictions and planning conditions can remove otherwise available rights.
Before paying for major works, ask your local planning authority or a suitably qualified professional to assess your exact proposal. A Lawful Development Certificate can provide useful formal confirmation where applicable.
Planning permission and building regulations are different
Planning considers the acceptability of the development. Building regulations focus on matters including structure, fire safety, insulation, ventilation and drainage. An extension that does not need planning permission may still need building regulations approval.
Budget for more than the application
Design drawings, measured surveys, structural engineering and building control can all add to the overall project cost. Planning application fees may change and depend on the application route; use the Planning Portal fee information for current official figures.
Working out whether the project is affordable?
Our calculator helps you explore a project budget alongside the planning process. It does not determine whether permission is required.
Calculate my extension budget →Planning FAQs
Can I build a 4m extension without planning permission?
Possibly in England, but it depends on the property, dimensions, site and all the restrictions. For many non-detached houses, a rear extension beyond 3m can trigger the larger-home-extension prior-approval process.
Can I apply after starting work?
Retrospective applications can sometimes be made, but permission is not guaranteed. Confirm requirements before work begins to avoid enforcement risk.
Does planning approval mean I can start building?
Not necessarily. Building regulations approval, party wall duties, conditions attached to consent and other permissions may still need attention.
General guidance only. Actual costs and permissions depend on the property, location and design. Seek project-specific advice before making financial or construction decisions.